This invention relates to a process and a device for producing elastane fibres from spinning solutions by a dry or wet spinning process using elastane recycling material, which optionally can be supplemented by admixing fresh solutions of elastane material in any proportions.
Elastane fibres are curtly produced mainly by two fundamentally different processes, the dry spinning process and the wet spinning process. In the dry spinning press, the spinning solvent of a polyurethane solution, typically in dimethylacetamide, is removed from the fibres spun in the spinning cabinet; the removal is effected by hot gas in the spinning cabinet and heating the spinning cabinet. In the wet spinning process for producing elastane fibres, the elastane solution to be spun is usually degassed, then prefiltered and transferred to a spinning tank. The polymer solution is subsequently filtered and, after optional further introduction of additives, the spinning solution is delivered to the spinnerets by means of metering pumps. This procedure is described, for example, by F. Founnxc3xa9e in Chemiefasern/Textilindustrie 44/96th Year, June 1994, page 365, or in U.S. Pat. No. 3,526,689.
By elastane solutions are meant solutions of polyurethanes or polyurethane ureas, which are conventionally made up of rigid and flexible segments, in suitable solvents such as dimethylacetamide or dimethylformamide. Usually polyester diols or polyether diols, depending on the intended use, are incorporated as flexible segments into the polyurethanes (polyurethane ureas).
Fresh spinning solutions, which are prepared by various solution processes, are generally used for producing elastane fibres. A practicable solution process is described, for example, in the document U.S. Pat. No. 3,428,711. Better solution processes have been disclosed by Offenlegungsschriflen DE 43 27 805 and DE 42 22 772 A1. Whereas, in the case of DE 43 27 805, one starts from a flow of liquid reaction materials, which are heated in a microwave cavity resonator in order to prepare an elastane solution, DE 42 22 772 describes a process for preparing elastane spinning solutions which have a stable viscosity and are low in gel by using a special multistep nozzle reactor. A diagram of the complicated preparation processes is reproduced in Chemiefasern/Textilindustrie 44/96th Year, June 1994, page 393, in FIG. 3.
Solution processes for elastane material in which the use of pure recycled material, i.e. for example, of already spun fibrous elastane material, are hitherto known only for a quite specific polymer composition.
According to the Canadian Patent CA 771086, spandex yams, which are produced if from metaxylylenediamine, a polyglycol and a diisocyanate, can be dissolved with a solvent, filtered and reprocessed to form elastane fibres by dry spinning through nozzles. For solution to be possible, the yams must have a linear polymer structure. In general, however, conventional elastane yams have cross-linked structures with a network in the form of biuret and allophanate bonding and consequently cannot easily be brought into solution
In another known process, according to the Japanese Offenlegungsschrift JP 56122836, solid pieces of elastane are dissolved in isopropyl alcohol together with chloroacetaldehyde and a strong acid such as, for example, sulfuric acid and the solutions formed are further used for the production of synthetic leather. A use for elastane fibres is not described. The document DD 143916 describes the preparation of polyurethane-polyurea solutions by addition of recycled polyurethane solution during the chain extending reaction. Solutions of that kind, however, are suitable only for synthetic leather production and not for producing elastane fibres. Besides these methods of utilising materials, so-called material recycling, methods of reusing raw materials, so-called chemical recycling, are also known (cf. E. Weigand; Plastverarbeiter 1995 46 (2) pages 88-92). In chemical recycling, the polyurethane material is converted back into diamine and polyether polyol, for example, by hydrolysis with superheated steam under pressure. Other possible ways of converting polyurethanes are provided by hydrogenation, pyrolysis or high-temperature gasification.
The reason why hitherto no recycling process for normal elastane waste for example, from yarn production, has been found lies in the fact that in all solution processes it is necessary to apply high solution temperatures, usually above 100xc2x0 C., in order to bring the elastane waste into solution. At these temperatures, however, the viscosity of the solution increases so much that no further processing can be carried out.
As in any elastane production process, waste arises in the form of residual fibres, for example, during disturbances in the course of production, during the spinning into other types and titres or as a result of defective batches, and as the raw materials price of the polymer material is high, as a rule more than 5.00 DM/kg polymer, the reuse of elastane waste is of great interest from the economic angle. The removal of fibrous waste (whether by dumping or by combustion) is likewise very cost-intensive, so that the provision of a process which renders possible the reprocessing of elastane waste is also very relevant from the environmental aspect.
The object is achieved by a process and a device for producing elastane fibres from spinning solutions by a dry or wet spinning process using elastane recycling material, which optionally can be carried out by admixing fresh elastane spinning solutions in any weight ratio, by adding secondary aliphatic amine, preferably diethyl amine, to the solutions.
The invention provides a process for producing elastane fibres by the dry spinning process or the wet spinning process using spinning solutions of elastane recycling material, optionally with the addition of fresh elastane solution, characterised in that
a) the waste obtained from elastane material, in particular from elastane fibres, is cut up, in particular to a cut length of at least 0.1 mm, preferably to a cut length of at least 1 mm, particularly preferably to a cut length of 2 to 10 mm, most preferably to a cut length of 3 to 5 mm,
b) the cut elastane material and 0.1 to 2 wt. %, based on the content of solid elastane, of a secondary aliphatic amine are introduced into the spinning solvent,
c) the mixture of cut elastane material, spinning solvent and secondary aliphatic amine is heated at a temperature of 60xc2x0 C. to 150xc2x0 C., with homogenisation,
d) the resulting homogeneous elastane spinning solution is prefiltered,
e) optionally with the addition of fresh elastane solution, the elastane spinning solution is mixed at 70xc2x0 C. to 140xc2x0 C. with a fresh elastane solution, preferably having the same solids concentration, in any mixing ratio and heating is continued,
f) the resulting spinning solution is cooled to a temperature of at not more than 70xc2x0 C., preferably from 50xc2x0 C. to 70xc2x0 C., and filtered once more,
g) the finished spinning solution is optionally remixed, for example, by stirring, degassed and subsequently spun into elastane fibres by the dry spinning process or the wet spinning process.
Preferably the elastane waste consists of both elastanes based on polyether and elastanes based on polyester, or of wastes of mixed polyether- and polyester-containing elastanes in any mixing ratio.
The secondary aliphatic amine used is in particular diethylamine (DEA), in a quantity preferably of 0.3 to 1 wt. %, in particular preferably of 0.5 to 0.8 wt. %, based on solid elastane.
The concentration of the finished spinning solution in step g) is from 22 to 45 wt. %, preferably from 30 to 40 wt. %.
Dimethylacetamide is preferably used as the spinning solvent.
In the case of a mixed spinning solution, the elastane spinning solution is mixed with a fresh elastane solution in step e) for a period of 5 to 60 minutes.
The total mixing time in steps b) and c) together is advantageously at least 10 minutes, preferably from 60 to 150 minutes, particularly preferably 90 to 120 minutes.
The solution temperature in step c) is advantageously from 80xc2x0 C. to 120xc2x0 C.
In the case where a mixture of waste elastane solution and fresh elastane solution is used, it is particularly advantageous to add the secondary amine, in particular diethylamine, to the fresh elastane solution not in step b) but in step f) and in the form of a stock batch of secondary amine and fresh elastane solution.
The recycling spinning solution and the fresh solution are preferably mixed together in a static mixer, preferably at 70xc2x0 C. to 140xc2x0 C., in particular at 100xc2x0 C. to 120xc2x0 C., for a period of 5 to 30 minutes and the spinning solution is subsequently cooled to 50xc2x0 C. to 70xc2x0 C.
The solids content of the recycling spinning solution in proportion to the solids concentration of the total spinning solution comprising recycling spinning solution and fresh elastane solution is preferably at least 10 wt. %, preferably at least 20 wt. %.
The spinning of the spinning solutions by the wet spinning process is carried out more effectively in particular from spinnerets having a nozzle hole diameter of 0.10 to 0.3 mm, preferably of 0.1 to 0.2 mm,
The elastane material in step a) is introduced preferably in portions, in particular via a metering/weighing device, preferably at a rhythm of 1 to 10 kg/minute, particularly preferably 3 to 5 k/minute, into the vortex created by an agitated disperser in the previously prepared spinning solution.
The invention also provides elastane fibres obtained from the process according to the invention, in particular with a titre of up to 10,000 dtex, their fibre strength being 0.5 to 0.95 cN/dtex, preferably 0.7 to 0.9 cN/dtex, and their elongation at tear being 500 to 750%, preferably 550 to 700%.
The invention further provides a device for carying out the process according to the invention, consisting at least of a heatable mixing tank with an inlet for solid material, a mixer and a dispersing unit, of a mixing zone with static mixing elements, connected downstream of the mixing tank, of a cooling zone with mixing elements and of a filtering unit for subsequently filtering the prepared elastane spinning solution.
The dispersing unit consists in particular of at least one, particularly two, agitated dispersers.
The agitated dispersers are preferably equipped with screw-type segmented appliances.
The screw-type segmented appliances are selected in particular from suction cutters, webbed rings, kneading spirals and multiple current appliances.
The mixer in the mixing tank is preferably an anchor mixer and the mixing tank is provided with an additional cleaning device for the internal fittings of the mixer.
Surprisingly, it has been found that cutting up elastane bobbin material and wool from the spinning cabinets into short fibres of in particular approximately 3 to 5 mm cut length in a cutting mill with suitable tools causes no difficulties. These short fibres, which have a density of approximately 0.2 g/cm3, can be conveyed flawlessly over spirals and screws without sticking and clogging, for example, from a silo into the mixing tank which is used for the preparation of the spinning solution.
Special mixing units are particularly helpful in the preparation of suitable spinning solutions from waste fibrous material. Experiments in a single-shaft agitated tank with various stirrer attachments showed that satisfactory results could not be obtained because of very long dissolving times and mixing times and very high specific driving powers owing to the high viscosity of the mixed material. It also became apparent that, in the agitated tank already containing spinning solvent (DMAC), with increasing addition of short elastane waste fibres the viscosity can be controlled and decreased only if secondary aliphatic amines such as, for example, diethylamine (DEA) are added to the spinning solution in the above-mentioned quantities of 0.5 wt. % DEA, based on solid elastane. Without the addition of secondary aliphatic amines, dissolving, mixing and heating lead to spinning solutions whose viscosity rapidly increased. Such spinning solutions can no longer be controlled technically. These spinning solutions are so highly viscous that they tend to gel and, owing to the high pressures required, they are also no longer conveyable. The required viscosity for the spinning process can then be adjusted, depending on the contact time, by heating, for example, the diethylamnine-containing spinning solutions to a temperature of up to 150xc2x0 C. The adjustment of the viscosity of highly concentrated elastane solutions by reacting the solution with secondary aliphatic amines in order to produce spinning solutions for the dry or wet spinning of elastane fibres has already been described in DE 44 46 339. It is surprising that secondary amines can now also be used for the direct working up of elastane material.
As already mentioned above, single-shaft agitated tanks with various stirrer attachments, even with the application of secondary aliphatic amines to the spinning solvent, alone do not suffice to prepare suitable spinning solutions from waste elastane fibres. Time and again this leads to sticking to the edge of the agitated tank and to clogging inside the agitated tank. Stated in other words, inhomogeneous spinning solutions which cannot be spun flawlessly into fibres are obtained, despite the addition of amine.
Unexpectedly, it has been found that these problems can be solved if one starts with mixing tanks having specific properties in order to prepare the spinning solutions.
A most particularly suitable mixing and dispersing machine has proved to be a unit which is equipped with double mantle for heating and cooling and at least one, preferably two, mixing shafts with dissolvers as well as a mixing shaft with anchor mixer (cf. FIG. 1). The mixing shafts are advantageously fitted with screw-type segmented appliances such as, for example, suction cutters, webbed rings, kneading spirals or multiple current appliances. To prevent sticky deposits of polymer on the inside wall of the tank which is in contact with the product, the anchor mixer is preferably equipped with spring-mounted cleaning segments.
By means of such a mixing and dispersing machine, by initially using spinning solvent (in particular dimethylacetamide) and secondary aliphatic amine and by adding short waste fibres, it is possible successfully to prepare elastane spinning solutions having solids contents of up to 40 wt. % which can subsequently be spun flawlessly into elastane fibres having good fibre properties.
It has proved to be advantageous to add the waste fibres via a metering/weighing device at a constant timed rhythm, in order to achieve a homogeneous solution process.
Furthermore, it has been found that it is particularly beneficial to introduce the short waste fibres as far as possible in the vicinity of the liquid vortices caused by the mixing shafts, in order to achieve a good wetting and solution of the fibres.
The solution process described is suitable for preparing spinning solutions which can be spun into elastane fibres by both the dry and the wet spinning process. The required spinning viscosity, which is distinctly higher in the dry spinning process than in a wet spinning process, can be adjusted via the amine content, the temperature of the solution and the residence time in the heater (cf. FIGS. 2 and 3).
In addition to spinning solutions from pure recycling material, mixtures with freshly prepared elastane spinning solutions can also be prepared in any mixing ratio, as described above.
The nature of the spinning solutions, whether they are, for example, spinning solutions which are by dissolving polyester- or polyether-containing fibrous material, or mixtures thereof in any composition, is irrelevant here (cf. Table 1). In all cases elastane fibres having good fibre qualities can be produced from such spinning solutions.
The elastane waste material (for example, fibres or wool) can contain up to 15 wt. % of conventional agents for fibre preparation, in particular, mineral oils or oils based on polyalkylsiloxane, for example, polydimethylsiloxane PDMS and/or ethoxylated PDMS or metal salts of fatty acids (for example, Mg stearate) or hydrotalcites alone or in any mixture.
The process using mixtures of fresh solutions and recycling material is particularly suitable for a continuous operation. The use of pure recycling solutions is particularly appropriate for a batch operation.
Elastane spinning solutions having a solids content of Up to 40 wt. % can easily be prepared using the devices and solution processes described (cf. Example 11, Table 1).
Besides the preparation of the spinning solution, the quality of the spinning solution is also decisive in the recycling process according to the invention. Good qualities in the spinning solution lead to readily filterable spinning solutions without rapid build-up of pressure in the filtering units and to long nozzle lives and consequently to high product yields.
It has been found that the proposed solution process can be combined particularly advantageously with a wet spinning process with the use of spinnerets with nozzle hole diameters of 0.10 to 0.3 mm, in particular 0.1 to 0.2 mm, in order to achieve long nozzle lives. With finer nozzle hole diameters, for example, less than 0.10 mm, a considerably increased build-up of the pressure of the spinning solutions in front of the nozzle may possibly occur, which leads to distinctly shorter nozzle lives. Nozzle hole diameters of more than 0.3 mm have extremely low spraying speeds (less than 5) and possibly lead to very long delays in spinning (more than 50), which results in unstable spinning processes and broken fibres during fixing.
The spraying speed (S) is found from:   S  =            4      ·              F        ⁡                  (                      m            ⁢                          /                        ⁢            min                    )                            Z      ·              d        2            ·      π      ·      100      
F=delivery (ccm/min)
Z=number of nozzle holes
d=nozzle hole diameter (cm)
The following Examples in association with FIGS. 1 to 3 serve to explain the invention further. All percentages given are based on the weight, unless indicated otherwise.